1. Field of the Invention
Provided is a novel process for preparing oligomeric alkanes. More specifically, an integrated process is provided where the alkanes are first dehydrogenated to create olefins, and then the olefins are oligomerized and hydrogenated to make the oligomeric alkanes. Transition metal catalyst complexes can be used in the integrated process.
2. Description of the Related Art
Olefins can be generated by direct dehydrogenation of alkanes with the removal of hydrogen gas or by the use of an acceptor such as ethylene to generate ethane. The chemical industry uses olefins as intermediates in a variety of processes. The largest chemical use is linear α-olefins in the formation of polyolefins such as ethylene-1-octene copolymers. Also and most importantly, low carbon number olefins have the potential to be converted into higher carbon number molecules that would be suitable for fuels, particularly, diesel. Other products formed from olefins include surfactants, lubricants, and plasticizers.
Iridium complexes as catalysts are known. During the 1980s, it was discovered that certain iridium complexes are capable of catalytically dehydrogenating alkanes to alkenes under exceptionally mild thermal (i.e., less than 160° C.) or even photolytic conditions (see, e.g., J. Am. Chem. Soc. 104 (1982) 107; 109 (1987) 8025; J. Chem. Soc., Chem. Commun. (1985) 1829). For a more recent example, see Organometallics 15 (1996) 1532.
In recent years, the chemical industry has employed the use of organometallic catalysts to produce polymers. While many advances in organometallic catalyst technology have been made, researchers continue to seek superior catalyst compositions. In fact, very recently, novel late transition organometallic catalysts have been discovered which are very effectively used in polymerization processes. More specifically, U.S. Pat. No. 6,037,297 to Stibrany et al., herein incorporated by reference, details group IB (Cu, Ag and Au) containing catalyst compositions that are useful in polymerization processes.
Organometallic catalyst technology is also a viable tool in oligomerization processes which produce linear α-olefins for use as feedstock in various other processes. However, one problem often encountered when using many of these catalyst systems is the propensity to produce α-olefins with very low selectivity (i.e., a Schulz-Flory type distribution with high k values). For instance, many of the linear α-olefins made today utilize a neutral nickel (II) catalyst having a planar geometry and containing bidentate monoanionic ligands. While these planar nickel (II) catalysts do produce linear α-olefins, these catalysis systems exhibit a Schulz-Flory type of distribution over a very wide range (i.e., C4-C30+).
To address the Schulz-Flory distribution problem, chromium metal based catalysts have become popular for use in certain oligomerization processes. More precisely, chromium complexes have been used to oligomerize ethylene in order to form linear α-olefins with improved distributions. In fact, there has been a report of a specific chromium catalyst which selectively trimerizes ethylene to 1-hexene. These techniques employ the use of a chromium compound in conjunction with aluminoxane along with one of a variety of compounds such as nitrites, amines and ethers. Unfortunately, while these techniques have been able to selectively produce α-olefins, polymer is formed as a co-product. Of course, when polymer is co-produced, the yield of desirable product decreases accordingly. Also, as a practical matter, polymer build-up in the reaction vessel can severely hamper production efficiency thereby limiting the commercial use of such processes.
As discussed above, the organometallic catalyst technology now being used to produce α-olefins has two major disadvantages. First, many of the organometallic catalysts produce α-olefins with a Schulz-Flory type distribution. Unfortunately, this Schulz-Flory type distribution is not ideal when short chain α-olefins are desired—in other words, the selectivity is not good enough to maintain efficient processes. Because α-olefins are used as intermediates for specific products, α-olefins with certain chain lengths are desired. For instance, the following are examples of α-olefin chain lengths that would be desirable as feeds for certain product types: C4 to C8 for comonomer in ethylene polymerization; C10 for lube quality poly-α-olefins; and C12 to C26 for surfactant products. Thus, considerable inefficiency and waste is present when significant amounts of α-olefins are produced having chain lengths outside of the range required for production of a particular chemical. Second, while some of the current organo-metallic catalysts may improve selectivity, most also produce polymer co-product. This lowers the yield of desired product and can also accumulate in the reaction vessel—both of which make commercial use less attractive and inefficient. Hence, there is still a need for improving the selectively and efficiency of linear α-olefin production.
U.S. Pat. No. 6,689,928 describes transition metal complexes and the preparation of oligomers using the complexes as catalysts. The starting material is an olefin, and overcomes the problems described above.
Improvements in the selectivity and efficiency of preparing oligomers, particularly alkane oligomers, are still needed. Processes which can improve the overall cost and economics of preparing oligomers would be of great benefit to the industry.